Eddy-current methods for non-destructive inspection are widely used in industry. Eddy-current methods can be used to measure the conductivity of conductive parts, as well as to detect cracks or flaws in such parts. A common application for the eddy-current method is to measure the conductivity of metal parts in order to ensure quality control. Eddy-current methods are particularly useful in the quality control of parts made of alloys, such as those of aluminum, in which deviations in the conductivity of the part indicates an improper mixture of alloyed metals, and/or incorrect heat treatment. Conductivity can also be used to determine the grade or temper of the alloy. Eddy-current conductivity testing is widely used in the manufacturing of aircraft components due to the widespread use of aluminum alloys.
In general, eddy-current testing works according to the following principle. An electrical coil is placed on the surface of a conductive material on which properties are to be measured. The coil is then excited using an alternating current. This produces an alternating magnetic field surrounding the coil that induces a circulating current or "eddy-current" in the material. In turn, the induced eddy-current induces a secondary voltage in the coil. The net effect of this secondary voltage is to change the impedance of the coil. The induced eddy-current, and thus the secondary voltage, is a function of the conductivity of the material.
Conductivity of non-ferrous metals is commonly specified as a percentage compared to copper. This unit is known as "%IACS" which stands for "Percent International Annealed Copper Standard." This standard is a hypothetical copper bar 1 m.times.1 mm.times.1 mm having a resistance of 1/58 ohm. Typically, eddy-current equipment is calibrated by detecting the impedance of the probe coil as the probe is applied to materials of known %IACS ("reference materials"). Any subsequent test material which produces an impedance in the probe similar to that caused by one of the known materials is assumed to have the same conductivity.
One of the major causes of error in conductivity measurements using the eddy-current method is known as "lift-off." Lift-off is the spacing between the electrical coil used to measure conductivity and the conductive material upon which measurements are to be taken. Lift-off can be caused by surface roughness, surface curvature, or a number of other factors which prevent the measuring coil from properly contacting the test material. The accuracy of the conductivity measurements can be increased by maintaining the probe and the conductive materials in a temperature-controlled oil bath prior to and during the conductivity measurements. This procedure maintains the probe and the conductive materials at a constant temperature, thus reducing any measurement errors due to temperature variations. However, the oil forms a film, that can vary in thickness, between the probe and the conductive materials. This thickness variation can also produce lift-off between the probe and the conductive materials.
Lift-off changes both the magnitude and phase of the impedance of the electrical coil used to measure the conductivity. This is important because some prior art methods use the magnitude of the impedance as an indication of conductivity while other methods use the phase of the impedance. In either case, lift-off results in an error in the subsequent observed value of conductivity associated with the measured impedance.
Some eddy-current systems use a balanced-bridge technique for making conductivity measurements. An electrical coil used to measure conductivity is connected in one arm of an electrical bridge. A variable resistor and/or capacitor are connected in the other arms of the bridge. The variable resistor and/or capacitor are used to balance the bridge and indicate the change in impedance. This method is capable of good accuracy, but is very time consuming to use. It is possible to reduce the effects of lift-off by adding an additional variable capacitor into the bridge circuit. This capacitor is adjusted, while the probe is placed on a typical conductive material, so as to make the bridge balance insensitive to lift-off. However, the adjustment is most effective only for materials whose conductivity is close to the conductivity of the material on which the adjustment was made. For other materials, the adjustment is only partially effective and the sensitivity of the system to lift-off is reduced but not eliminated. Therefore, in order to ensure that the effects of lift-off are eliminated, the bridge must be readjusted when a material whose conductivity is outside a limited range of conductivity is tested. Furthermore, inaccurate results are obtained if the adjustment is not done properly.
Other eddy-current systems use a digital inductance-capacitance-resistance (LCR) meter with high accuracy to measure the impedance of the electrical coil. The digital LCR meter is connected to a computer through an electrical interface. These systems rely on careful probe placement to minimize lift-off.
Thus, a number of methods have been devised to try and reduce the effects of lift-off. These methods are capable of achieving accurate results; however, the methods are complex and an operator that is unskilled or inattentive often fails to achieve accurate measurements. As a result, defective parts pass through inspection without being detected.